Compact and efficient elastocaloric heat pumps—Is there a path forward?

Elastocaloric cooling holds promise for energy-efficient heat pumping near room temperature with low environmental impact. Its adoption is, however, impeded by disproportionally large sizes of actuators compared with the active material volume. Taking magnetocaloric cooling as the baseline, the value of no more than 10:1 actuator volume to active material volume should lead to both size- and cost-effective solutions that may potentially be competitive with vapor-compression devices. With the goal to establish performance metrics that can lead to informed actuator selection for specific regenerator requirements, we analyze a wide range of elastocaloric materials and actuator technologies to find the best matches. We find that actuation with magnetic shape memory alloys meets all requirements; however, this technology is currently in early developmental stages and such actuators are not widely commercially available. Another promising and easily accessible option is standard rotary electric motors in combination with rotary-to-linear transduction mechanisms. A theoretical analysis of two case studies of elastocaloric systems using rotary electric motors with a Scotch yoke mechanism demonstrates the usefulness of our approach. Actuator requirements are based on two different regenerator configurations: one starting from zero strain without any mechanical energy recovery and another with 2% pre-strain and mechanical energy recovery to reduce the power and torque required from the motor. Our results indicate that the 10:1 target actuator to active material volume ratio can be met and feasibly lowered further, demonstrating that the proposed method for selecting actuators makes compact and efficient elastocaloric systems possible

STUDY INTO THE MAGNETIC FIELD FOR A MAGNETOCALORIC COOLING SYSTEM WITH THE USE OF MAGNETOVISION

The paper presents measurements of the magnetic field intensity of a magnet used to generate a magnetic field in a magnetocaloric cooling system. The magnet – a Halbach array consisting of several permanent magnets – is cylindrical with a hole in the centre. The special arrangement of these magnets concentrates the magnetic field in the inner gap. The generated magnetic field was examined with the aid of a magnetovision system designed in our laboratory. The investigations confirmed that a magnetic field of 1 T is produced in the hole. In the magnetic pictures one can observe how the direction of magnetic field vector is changing. In addition, the influence of magnetic field screen on the magnetic field range was analysed

CaloriSMART: Small-scale test-stand for rapid evaluation of active magnetic regenerator performance

We report operation of a device designed specifically for rapid experimental evaluation of performance of magnetocaloric materials in different magnetic fields using a compact active magnetic regenerator bed with a total volume of approximately 5 mL. Other features of the system include digital control of the rotating-permanent-magnet field source and custom dual-opposed syringe pump that enable precise tuning and coupling of the flow profile and the magnetic field profile. Performance of the device is demonstrated for flow volumes between 1 and 4 mL (utilization from 0.48 to 1.9), maximum magnetic fields of 1.13 and 1.45 T, and applied cooling powers from 0 to 20 W at frequencies from 0.5 to 4 Hz. A regenerator comprised of 25 g of 200 µm spherical Gd powder reached temperature spans of 19.3 K under no applied cooling load and 2.6 K under the maximum applied cooling load of 20 W. The device also achieves a very high specific exergetic cooling power of 73 W L−1 T−1. Results obtained at two different maximum magnetic fields in the same device suggest a powerful new scaling for regenerator performance: the exergetic power quotient. The exergetic power quotient shows a simple scaling of device cooling performance with the amount of active material and the magnetic field strength. This suggests results from a small device correlate to expected performance of a larger regenerator, making the exergetic power quotient a well-suited parameter for evaluating functionality of active magnetic regenerators employing new magnetocaloric materials

Low-force compressive and tensile actuation for elastocaloric heat pumps

The elastocaloric effect underpins a promising solid-state heat pumping technology that, when adopted for commercial and residential applications, can revolutionize the cooling and heating industry due to low environmental impact and substantial energy savings. Known operational demonstration devices based on the elastocaloric effect suffer from low endurance of materials and, in most experimental systems, from large footprints due to bulky actuators required to provide sufficient forces and displacements. We demonstrate a new approach which has the potential to enable a more effective exploitation of the elastocaloric effect by reducing the forces required for actuation. Thin strips of NiTi were incorporated into composite structures with base polymer, such that bending the structures results in either exclusively compression or exclusively tension applied to the elastocaloric strips. The structures allow compression of thin elastocaloric strips without buckling, realize more than 50 % reduction in required forces for a given strain compared with axial loading, and open up a wide range of possibilities for compact, efficient elastocaloric devices

Balancing performance of active magnetic regenerators: a comprehensive experimental study of aspect ratio, particle size, and operating conditions

Effective and, at the same time, efficient active magnetic regenerator (AMR) performance requires balanced geometry and operating conditions. Here the influence of regenerator shape, magnetocaloric material size, operating frequency, and utilization on the performance of gadolinium packed-particle bed AMRs is demonstrated experimentally. Various metrics are applied to assess effectiveness and efficiency. Observed temperature spans and cooling powers across a wide range of operating conditions are used to evaluate system performance and estimate exergetic cooling power and exergetic power quotient. A new metric combining exergetic cooling power and pump power provides an estimate of the maximum achievable second law efficiency. Five regenerator geometries with equal volumes and the aspect ratio from 1.0 to 3.8, and four different ranges of Gd spherical particles between 182 and 354 µm, are investigated. Improvements in system performance are demonstrated by a boost in specific cooling power of gadolinium from 0.85 to 1.16 W g−1 and maximum temperature span from 8.9 to 15.1 K. The optimum exergetic cooling power is observed for 1.37 utilization and 3 Hz operating frequency, exergetic power quotient exhibits a maximum at the same utilization but at 2 Hz frequency, while the highest efficiency is recorded at 1 Hz and utilization of 0.5, demonstrating that multiple performance metrics must be balanced to achieve regenerator design meeting all performance targets.This article is published as Czernuszewicz, Agata, Lucas Griffith, Julie Slaughter, and Vitalij K. Pecharsky. "Balancing performance of active magnetic regenerators: A comprehensive experimental study of aspect ratio, particle size, and operating conditions." 5 Journal of Physics: Energy (2023): 024008. DOI: https://doi.org/10.1088/2515-7655/acc1a0. Copyright 2023 Attribution 4.0 International (CC BY 4.0). Posted with permission. DOE Contract Number(s): AC02-07CH1135

Active magnetic regenerative cooling with smaller magnets

Magnetocaloric heat pumping near room temperature, a.k.a. magnetic cooling, relies on the active regenerator cycle to achieve functional temperature spans and realize economic and societal benefits promised by this emerging solid-state cooling technology. The cycle itself depends upon synchronizing oscillating flow of a heat transfer fluid through a solid porous active material matrix, or the refrigerant, with periods when the refrigerant is in the highest available (field-on) and in nearly zero (field-off) magnetic fields to accomplish heat transfer. With this in mind, we analyze varying flow and magnetic field wave forms and the timing between when the fluid is pumped and when the magnetic field is turned on and off. We demonstrate that the volume and the cost of permanent magnet generating the field changes can be cut nearly in half with little to no effect on the device temperature span and cooling power normalized by the refrigerant mass.</p

Balancing performance of active magnetic regenerators: a comprehensive experimental study of aspect ratio, particle size, and operating conditions

Effective and, at the same time, efficient active magnetic regenerator (AMR) performance requires balanced geometry and operating conditions. Here the influence of regenerator shape, magnetocaloric material size, operating frequency, and utilization on the performance of gadolinium packed-particle bed AMRs is demonstrated experimentally. Various metrics are applied to assess effectiveness and efficiency. Observed temperature spans and cooling powers across a wide range of operating conditions are used to evaluate system performance and estimate exergetic cooling power and exergetic power quotient. A new metric combining exergetic cooling power and pump power provides an estimate of the maximum achievable second law efficiency. Five regenerator geometries with equal volumes and the aspect ratio from 1.0 to 3.8, and four different ranges of Gd spherical particles between 182 and 354 µ m, are investigated. Improvements in system performance are demonstrated by a boost in specific cooling power of gadolinium from 0.85 to 1.16 W g ^−1 and maximum temperature span from 8.9 to 15.1 K. The optimum exergetic cooling power is observed for 1.37 utilization and 3 Hz operating frequency, exergetic power quotient exhibits a maximum at the same utilization but at 2 Hz frequency, while the highest efficiency is recorded at 1 Hz and utilization of 0.5, demonstrating that multiple performance metrics must be balanced to achieve regenerator design meeting all performance targets

CaloriSMART: Small-scale test-stand for rapid evaluation of active magnetic regenerator performance

We report operation of a device designed specifically for rapid experimental evaluation of performance of magnetocaloric materials in different magnetic fields using a compact active magnetic regenerator bed with a total volume of approximately 5 ml. Other features of the system include digital control of the rotating-permanent-magnet field source and custom dual-opposed syringe pump that enable precise tuning and coupling of the flow profile and the magnetic field profile. Performance of the device is demonstrated for flow volumes between 1 and 4 ml (utilization from 0.48 to 1.9), maximum magnetic fields of 1.13 and 1.45T, and applied cooling powers from 0 to 20w at frequencies from 0.5 to 4hertz. A regenerator comprised of 25g of 200um spherical Gd powder reached temperature spans of 19.3k under no applied cooling load and 2.6k under the maximum applied cooling load of 20w. The device also achieves a very high specific exergetic cooling power of 73WL-1 t-1. Results obtained at two different maximum magnetic fields in the same device suggest a powerful new scaling for regenerator performance: the exergetic power quotient. The exergetic power quotient shows a simple scaling of device cooling performance with the amount of active material and the magnetic field strength. This suggests results from a small device correlate to expected performance of a larger regenerator, making the exergetic power quotient a well-suited parameter for evaluating functionality of active magnetic regenerators employing new magnetocaloric materials.</p

Low-force compressive and tensile actuation for elastocaloric heat pumps

The elastocaloric effect underpins a promising solid-state heat pumping technology that, when adopted for commercial and residential applications, can revolutionize the cooling and heating industry due to low environmental impact and substantial energy savings. Known operational demonstration devices based on the elastocaloric effect suffer from low endurance of materials and, in most experimental systems, from large footprints due to bulky actuators required to provide sufficient forces and displacements. We demonstrate a new approach which has the potential to enable a more effective exploitation of the elastocaloric effect by reducing the forces required for actuation. Thin strips of NiTi were incorporated into composite structures with base polymer, such that bending the structures results in either exclusively compression or exclusively tension applied to the elastocaloric strips. The structures allow compression of thin elastocaloric strips without buckling, realize more than 50 % reduction in required forces for a given strain compared with axial loading, and open up a wide range of possibilities for compact, efficient elastocaloric devices.</p